smb_copymsgmem() fix:

Did not clear/set convenience pointers (e.g. msg.subj), so they would point
to the source message hfield_dat buffers which may be freed or garbage.
Apparently nothing that used smb_copymsgmem() used the convenience pointers
(before now), so it hasn't been a problem before now. <shrug>
It became a problem becaue I'm going to use this function in js_put_msg_header
to fix a problem where the underlying smbmsg_t representation is not current
even after MsgBase.put_msg_header() is used.